The AtomChip: Bringing Nanofabrication and Quantum Optics Together

One of the scientific revolutions of the 20th century is Quantum
Mechanics. It is a weird theory, extremely different from our day to day
experiences, in which, for example, a particle can act as a wave or can be in
several places at once.

It is also very different from our deterministic classical laws of physics as
it is probabilistic in nature. This strange theory has already enabled marvels
of technology such as the atomic clock.

One of the technological revolutions of the 20th century is the
electronic integrated circuit. Such circuits, in which millions of transistors
are put into a small semi-conductor chip, have enabled the computer and most
technological gadgets that today engulf our lives.

It is not uncommon that joining two seemingly unconnected fields of knowledge
brings about dividends that reach far beyond the mere sum of the potential
embedded in the two parent disciplines.

The AtomChip is such a story. It brings together the best of both worlds: the
relatively mature field of micro- and nano- fabrication and the new set of
scientific rules provided by quantum theory. Together they form quantum
technology, with the promise of devices such as miniature atomic clocks,
magnetic sensors, inertial navigation systems, gravitation field sensors,
quantum communication and cryptography, and the quantum computer.

The main idea is to make possible the co-existence of a quantum system within
a classical environment, so that the classical environment enables effective
control of the quantum system as well as effective information exchange by
standard devices such as present day electronics and computers.

The main challenge comes from the fact that while we want to couple to the
quantum system for control and information exchange, quantum systems survive in
their quantum state only under severe conditions of isolation.

The first generation of AtomChips were designed and operated at the turn of
the century [1,2]. These chips were based on
micro-fabricated electrodes holding currents and charges, and on a quantum
system in the form of ultra-cold neutral atoms cooled by lasers and other simple
means without the need for a cumbersome cryogenic apparatus. These simple
cooling methods gave rise to Nobel prizes in 1997 and 2001.

Thus, neutral atoms were trapped in vacuum a few microns above the surface of
a room temperature chip. If, in a semi-conductor chip, the system of interest
moves within the chip, here, the system of interest is trapped and guided within
electric, magnetic and electro-magnetic fields, microns above the chip.

Figure 1. An AtomChip for neutral atoms with which a
previously unknown phenomenon in electron transport was observed [5] (collaboration with Joerg Schmiedmayer).

Today the chip has evolved into different formats. For example, in addition
to neutral atoms, cold isolated ions, molecules and even electrons are trapped
above the surface. To increase simplicity, capsules with hot atomic vapor are
also used.

In addition, attempts are being made to utilize solid-state lattices which
exhibit the unique feature of not destructively coupling to a quantum system
embedded within them [e.g. Nitrogen-Vacany (color) centers within a carbon
lattice (diamond)].

In numerous laboratories as well as private companies, AtomChip technology is
currently being simplified and miniaturized. The vacuum chamber will, for
example, be located inside the silicon substrate. Integrated diode lasers and
photonics will provide efficient light-matter interaction for delicate
information exchange.

We are thus not far from the point in time when AtomChips may be embedded in
standard electronic boards alongside 20th century electronic
components. Externally, one will not be able to tell them apart.

For those familiar with fabrication, it is interesting to note that the AtomChip has many completely new figures of merit in terms of quality and yield of chips. It also requires new materials and geometries. For example,
while surface or edge roughness of conducting wires are of little concern in the
semi-conductor industry (as long as conductivity remains high), quantum optics
requires extreme smoothness.

Furthermore, while electrically anisotropic materials are nowhere to be found
in the electronics industry, they have been shown to reduce hindering effects
for quantum optics by orders of magnitude.

Similarly, while in conventional processes contamination is avoided at all cost, it has been shown that for quantum optics, contamination may be advantageous. The
interested reader can learn more about the state-of-the-art in the fabrication
of AtomChips in [3].

If one is interested in the specific example of quantum computing, a good
overview is given in the special issue of the journal Quantum Information
Processing, about to be published [4].

At Ben-Gurion University of the Negev (BGU) we have constructed one of the
first, if not the first, fabrication facility designed from the start to address
the needs of the AtomChip community.

In the figures, I present two chips fabricated at BGU. The first is an AtomChip for neutral atoms, which featured in the Science magazine in 2008, and which due to the ultra sensitivity to the atom-surface interaction, enabled the observation of a completely unknown phenomenon in electron transport. A tiny cloud of a few thousand neutral atoms trapped microns above the electrodes of an AtomChip is also presented.

The second AtomChip is adapted for charged atoms. Two ions trapped on this chip microns above the surface are shown in the figure. Finally, I present a schematic view of how the future AtomChip will be structured.

Figure 2. Figure 2. A dilute cloud of a few thousand ultra-cold atoms a few micron-meters above the surface of an AtomChip. The electrodes of the chip are visible in the background.Taken from [6].

Figure 3. Fluorescence from two ions trapped micron-meters above the surface of the AtomChip shown below in figure 4 (collaboration with Ferdinand Schmidt-Kaler).

Figure 4. AtomChip for charged atoms (Ion Chip) fabricated at BGU and ready to be put into the chamber in Mainz, Germany.

Figure 5. A schematic view of how a future AtomChip device would be structured. The miniature vacuum chamber will be embedded into the silicon substrate. The chip would integrate all required particle/light sources as well as MEMs valves, photonics, high-Q resonators, and readout via fibers and electronics (courtesy of Tim Freegarde)

The AtomChip is not only a mind-boggling example of synergy between two
disciplines. It is also a wonderful example of the integration of many different
operational elements in one monolithic device: metallic electrodes for currents
and charges side by side with photonics and high-Q resonators, MEMs, lasers, and
so on.

Further, the AtomChip platform may enable the integration of several
different quantum systems. These so-called hybrid quantum systems may include,
for example, logic gates made of superconducting qubits (quantum-bits) and
quantum memory in the form of trapped atoms.

As Science is after all about knowledge and not only technology, it is
perhaps fitting to end this brief outline with a broader and somewhat
philosophical point of view. As the AtomChip enables more and more complex
quantum operations, it will also enable deeper and deeper insights into quantum
theory. Such understanding of one of the strangest realms of nature may have
deep implications concerning our concept of the universe as well as our
perception of ourselves. For example, answering the enigma of whether brain
functions follow quantum logic or classical logic may have consequences
concerning the question of free will.

Whatever the future holds for the AtomChip, it is quite clear that this chip
has invited us for quite a ride.